The invention relates to a electronic circuit comprising: a two-port network with variable conductance between a pair of input terminals for receiving an input voltage and a pair of output terminals of the two-port network, the two-port network comprising:
a first transistor having a conduction channel connected between a first input terminal of the pair of input terminals and a first output terminal of the pair of output terminals, and having a control electrode arranged to receive a control voltage to control the conduction of the conduction channel of the first transistor, PA1 a second transistor having a conduction channel connected between a second input terminal of the pair of input terminals and a second output terminal of the pair of output terminals, and having a control electrode arranged to receive a control voltage to control the conduction of the conduction channel of the second transistor, and PA1 means for supplying the control voltage to the control electrode of the first transistor and the control electrode of the second transistor. PA1 a first signal follower having an input connected to the first input terminal and having an output, PA1 a second signal follower having an input connected to the second input terminal and having an output, PA1 a resistor ladder connected between the output of the first signal follower and the output of the second signal follower, which resistor ladder has respective nodes between successive resistors connected to the respective control electrodes of the series-connected individual transistors, and PA1 a first controllable current source connected to a first node of the resistor ladder and second controllable current source connected to a second node of the resistor ladder. PA1 two further series arrangements of at least two individual transistors whose conduction channels are arranged in series, one of the two further series arrangements being connected between the first input terminal and the second output terminal and the other one of the two further series arrangements being connected between the second input terminal and the first output terminal, and the means for supplying the control voltage further include:
Such an electronic circuit is known from U.S. Pat. No. 4,509,019, which shows active RC filter circuits comprising balanced amplifiers to which pairwise arranged filter resistors and filter capacitors are connected. To enable the filter circuit to be tuned the filter resistors are formed by MOS transistors whose channel resistance is adjusted by means of a variable voltage on the control electrodes of the MOS transistors. The MOS transistors are arranged in pairs in two-port networks, one MOS transistor being connected between one of the input terminals and one of the output terminals of the two-port network and the other MOS transistor between the other input terminal and the other output terminal. The two-port networks are combined with one or more differential amplifiers having differential inputs and outputs. Depending on their use the two-port networks function as a variable resistance between a balanced signal source and the differential inputs of a differential amplifier, or as a variable feedback resistance between the differential inputs and outputs of a differential amplifier. Thus, active tunable higher-order RC filter circuits can be realized by means of a plurality of differential amplifiers, two-port networks and capacitors arranged in pairs. The pairwise arrangement of the MOS transistors and the balanced signal input largely eliminates the distortion owing to mismatch between the MOS transistors and owing to the non-linear relationship between the channel current and the gate voltage of a MOS transistor.
The MOS transistors are operated in the non-saturation region of their voltage-current transfer characteristics, the resistance of the conduction channel of the transistor being controlled by means of a voltage on the control electrode or gate of the transistor. The progress in technology has resulted in MOS transistors with increasingly thinner gate oxide being available to the designer. A direct consequence of this is that the gate voltage swing between the threshold voltage of the MOS transistor and the voltage at which the channel conduction no longer increases significantly is much smaller than it used to be. Now a problem arises if such a MOS transistor is used as a controllable conductance in the afore-mentioned balanced active RC filter circuits. The voltage range over which the MOS transistor can be used as a controllable conductance in such RC filter circuits is linked to the gate voltage swing. As a result of the reduced gate voltage swing the distortion will increase comparatively rapidly when the signal level remains the same and, consequently, the usability of active RC filter circuits of the afore-mentioned type will decrease.